Mark IntRange::iterator::operator{==,!=} as const. (swiftlang#28390)

`IntRange::iterator::operator==` must be marked as `const` in order to use
`OptionalTransformRange<IntRange<...>, ...>::empty` under specific conditions.

Author: dan-zheng

include/swift/Basic/Range.h

@@ -126,8 +126,8 @@ class IntRange {
       *this -= 1;
       return copy;
     }
-    bool operator==(iterator rhs) { return Value == rhs.Value; }
-    bool operator!=(iterator rhs) { return Value != rhs.Value; }
+    bool operator==(iterator rhs) const { return Value == rhs.Value; }
+    bool operator!=(iterator rhs) const { return Value != rhs.Value; }
 
     iterator &operator+=(difference_type i) {
       Value = Traits::addOffset(Value, i);